The present invention relates to a method for damping chatter oscillations in a processing machine, in particular a cutting machine, and to a device for carrying out the method.
Chatter oscillations in a workpiece or in a tool can occur when materials, particularly metals, are cut with a machine tool. Chatter can produce unusable surfaces and waste. Frequently, chatter occurs when the machine structure mechanically yields under the applied cutting forces. Periodic excursions are observed in particular when the cutting forces are excited at a frequency close to one of the characteristic resonance frequencies of the machine. These periodic excursions due to chatter can cause periodic discontinuities in the cutting force which under certain phase relationships with the machine resonances can sustain and/or even amplify chatter. The presence of chatter oscillations limits machine productivity, in particular when materials that require a high cutting force or a large cutting depth are cut. Chatter may only be reliably eliminated by reducing the cutting depth below a certain value.
If the desired cutting depth is to be maintained while eliminating chatter, the machine structure has to be either stiffened or better damped. Frequently, the available installation space and/or the weight or the costs of the machine make it difficult to implement a stiffer construction. Damping is difficult to improve by employing only mechanical means. The materials used in the construction of the machine have only very small and unpredictable intrinsic damping, in the order of a few percent.
The publication “Hochgenaue Regelung von Linearmotoren durch optimierte Strommessung” (High Precision Control Of Linear Motors Through Optimized Current Measurements) published in the German technical journal “antriebstechnik”, Vol. 38 (1999), No. 9, pp 90-93, discloses a feed system with a permanent-excited synchronous linear motor and a field control with a high-resolution PWM transistor converter and a synchronized, high precision current measurement. A conventionally controlled linear motor with low friction guides exhibits under controlled operating conditions a parasitic motion which is superimposed on the feed motion. This parasitic motion is also observed when the motor is stopped. An adequate motion quality can be achieved by measuring the current with a secondary current controller while controlling the position. With a closed loop control, noise produces a corresponding feed force in the linear motor, which then causes a parasitic motion of the feed carriage. Only the parasitic components of the current along the q-axis (quadrature or out-of-phase axis), where the force is produced, cause a parasitic force and hence a parasitic motion. The parasitic components of the current along the d-axis (direct or in-phase axis), where the field is formed, do not affect the parasitic motion. Due to the high inertia of the carriage, high parasitic frequencies have only a small effect on the position of the carriage. The parasitic frequency curve has a maximum at intermediate frequencies, depending on the control bandwidth of the velocity and position control. This is the frequency range where disturbances in the current measurement have the greatest impact on the position of the carriage. A precise feed motion can be realized with a single drive system, which includes a synchronous linear motor, by synchronously measuring the currents, for example, by using an oversampling method with an effective resolution of 12 bits. This high-precision current measurement in conjunction with a field control improves the parasitic motion by a factor of 20, using the same control dynamics.
Unlike rotary servo motors, linear motors used for driving feed axes have a flat air gap. Linear motors have a (feed) direction along which the feed force is generated, and another (force) direction along which the attractive magnetic force is produced. The feed direction is parallel to the plane of the air gap, whereas the force direction is oriented normal to the plane of the air gap. Because the attractive force is perpendicular to the drive force, this force is also referred to as transverse force. In principle, the linear motor can produce controllable forces both in the feed direction and also in the transverse direction. For controlling the feed force, the q-component (quadrature or out-of-phase component) of the three-phase current is used, whereas the d-component (direct or in-phase component) is responsible for the attractive force. The two components are perpendicular in a three-phase system. Controlling the drive force via the q-component of the motor current does not affect the attractive force and vice versa. The two force directions are hence decoupled from each other. In conventional machine tools, only the direction of the feed force, i.e., the q-component is operational, because the machine carriage moves in the direction of the feed force. The attractive force is not controlled in conventional machine tools, so that the current of the d-component is always set to zero.
It would therefore be desirable and advantageous to provide an improved method and device for damping chatter oscillations in a machine tool, which obviates prior art shortcomings and is able to specifically operate with a linear motor controlled with a single field controller.